Haldor Topsoe, a global leader in catalyst manufacture, has recently shared insight on the value of using powder rheology to assess the feasibility of selecting a fluid diaphragm pump to transport a given powder. The company realizes savings of ~£40,000 each time a pump is successfully specified in place of a more costly pneumatic conveying system and estimates that over £250,000 has been saved to date. This represents a very healthy return on investment for the associated powder testing instrumentation (FT4 Powder Rheometer®, Freeman Technology, Tewkesbury, UK.)
Fluid diaphragm pumps are a cost-effective choice for powder transport with installed costs in the order of 30% of a pneumatic conveying system. However, they are not suitable for all powders. Setting a specification that securely differentiates powders that will pump efficiently and consistently is essential to ensure successful implementation of the lower-cost solution.
Haldor Topsoe has defined a robust specification that involves measuring three powder properties:
Compressibility: the extent to which a powder sample changes in volume (density) when subject to consolidating stress.
Permeability: the extent to which a powder sample resists the passage of air, which can be determined from measurements of the pressure drop across a powder bed as a function of air velocity, at a defined consolidation pressure.
Cohesion: a shear property, determined by shear cell testing.
Compressibility is the most powerful differentiator of pump performance and is used as a primary screen. Higher compressibility is typically indicative of greater cohesivity which is widely recognized as potentially detrimental to powder pumping performance. More cohesive powders pack relatively inefficiently, trapping air which is forced out through the application of consolidation stress. This causes a significant change in bulk density associated with higher compressibility.
If compressibility is lower than 36% then a fluid diaphragm pump is suitable; above 38% a pneumatic conveying system is required. Permeability and cohesion can be used to differentiate powders with intermediate compressibility and similar equipment selection criteria have been defined. Bulk density values, generated as a by-product of compressibility measurements, are used to define system capacity. This simple approach of testing representative powders and correlating performance with the operational plant can be readily adopted by any company facing a similar challenge of selecting optimal powder handling solutions for a range of powders. Once such an approach has been established cost-effective systems can be specified with confidence and ease.
Freeman Technology specialises in systems for measuring the flow properties of powders and has over a decade of experience in powder flow and powder characterisation. The company invests significantly in R&D and applications development, and provides detailed know-how alongside its universal powder tester, the FT4 Powder Rheometer. Expert teams ...